mystery wire

Im looking to tap into an oversized junction box with an outlet in my basement. The supply wire going to it is pretty heavy gauge with a white, black, ground, red and blue! The pos and neg are pigtailed to the outlet and the blue and red are capped off. Nothing is plugged into the outlet but it is located above my hot water heater. No other wires go to it or from it. I would like to know what this is before I tap into it, any help? Thanks!

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mystery wire

Check the blue and red wires with a volt meter. They are probably hot and that is why someone capped them. I suspect that if you checked your main panel you would find a blue wire attached to a breaker. The red wire in question would probably be harder to find on your panel since there are most likely several red wires attached to breakers.

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mystery wire

Measure all combinations including red-white, blue-white, and red-blue. You are likely to find (respectively):

120, 120, 208
120, 208, 240 (or 208, 120, 240)

You may not tap off of that cable to feed ordinary receptacles without first going through a (perhaps newly installed) subpanel with 15 and/or 20 amp. breakers or unless the breakers serving that cable are no more than 20 amps which defeats (and wastes) the use of the fat conductors.

If this is a 3-phase circuit's cable, you are not obliged to use all 3 phases in a subpanel. Particularly if one of the phases has a higher voltage (like 208) to neutral, you might cap that one off and use just the other two.

Do not use 208 volts for appliances that do not specfically accept that.

If your home has a second meter for the water heater, you are not supposed to tap off of that feed (red and blue?) for anything else.